The present invention relates generally to an apparatus for shielding a circumferential zone about tubes, pipes, rods or other elongated workpieces during welding or heat treatment. More particularly, this invention concerns an improved gas shielding system for welding which incorporates two chambers, one opening annularly adjacent to and in a direction across the thermal zone and the other opening cylindrically about the workpiece in a direction away from the zone, with each being connected to independently controlled sources of inert gas, to avoid air contamination.
Welding or heat treatment involves application of localized heating to a work piece, or to a joint between adjacent work pieces, sufficient to accomplish the intended result. Such heating typically takes place in the surrounding air, which can be a source of contamination for the region being treated due to the dynamics of the process. Shielding with inert gas is adopted wherever process stability and compositional control of the metal alloys involved are necessary. For example, when arc welding with inert gas is contaminated with air, arc behavior can become erratic and excessive oxidation of the deposit can occur. These can affect the weld deposit's soundness or porosity, geometrical consistency, mechanical properties, and/or corrosion resistance. To effectively prevent either migration and/or aspiration of air to the intensely heated region (e.g. welding, heat treating), shielding of the region in a controllably uniform manner (i.e. with laminar-like flow and even coverage) is desirable.
The importance of shielding to obtain high quality welded or heat treated areas has long been recognized, and various shielding arrangements have been available heretofore. For example, U.S. Pat. No. 2,903,559 to Wempe shows a torch for arc welding with inner and outer concentric nozzles surrounding the electrodes which deliver protective gases about the weld zone to define two protective zones. Similarly, U.S. Pat. No. 2,868,950 to Gage discloses a metal arc welding process and apparatus including two annular gas passages which produce protective gas shields about the welding arc. Both the Wempe and the Gage Patents are thus representative of the prior art approach to shielding, which is to discharge suitable shielding or inert gas through annular passages surrounding the welding nozzle. The more recent U.S. Pat. No. 4,000,392 to Banas relates to a laser welding method wherein a layer of shielding gas is maintained to the weld zone. Each of these patents is illustrated as being particularly applicable to flat workpieces, as opposed to tubular or cylindrical workpieces.
The welding or heat treatment of cylindrical or tubular workpieces, such as sections of tubing or the like, involves some additional factors which complicate the problem of providing effective shielding against air contamination. Precision tubing, like that utilized in nuclear power plants, is typically welded in fixtures wherein the tubing is rotated past a welding nozzle or laser. The high temperature of the welding region near the surface of the tubing causes a decompressed region which tends to draw air toward it. In addition the viscous boundary layer effects associated with the rotating tubing tends to pull air toward the heat source. Air can thus enter the process zone by aspiration, transpiration, and diffusional migration.
Complete enclosure of the tubing, support fixture and welder within a chamber of inert gas is impractical, and the shielding devices of the prior art are neither adapted or suited for high reliability circumferential welding or heat treatment in a compact and efficient manner. The prior devices have suffered from the disadvantages of poor shielding gas economy, low controllability and reliability, and limited interchangability between the various welding processes. In addition, some special metal alloys such as titanium and zirconium are especially sensitive to air contamination during welding, and the prior shielding techniques have been inadequate for use with such alloys.
A need has thus developed for an improved shielding system whereby welding and heat treatment of rotating elongated workpieces such as tubing and the like, can be accomplished in a more compact, efficient, and reliable manner with better results.